Transcription and Translation - Protein Synthesis From DNA - Biology

In this video, we're going to talk about transcription and translation.

And here is just a basic overview of these two processes.

Transcription is the process by which DNA gets converted into mRNA, also known as messenger RNA.

And translation is the process of converting the information stored in messenger RNA and using it to build protein.

Now, if you recall, which organelle is used to make proteins?

What would you say?

The answer is the ribosome. The ribosome is the site at which proteins are manufactured in the cell.

So we're going to talk about that shortly, but let's go over transcription first before we talk about translation.

Transcription occurs in three steps: initiation, elongation, and termination.

Now, during initiation, RNA polymerase binds to the promoter region of DNA.

Now, what is the promoter region?

The promoter region is basically a short sequence of DNA.

In eukaryotic DNA, it's T A T A A A, also known as the TATA box.

Now, this sequence is located 25 nucleotides upstream of the site where transcription begins.

The next thing that RNA polymerase does is it causes the two DNA strands to separate.

And during elongation, it begins to add nucleotides to the growing mRNA strand that we see here.

Thus, RNA polymerase synthesizes mRNA starting from the five prime end

going to the three prime end.

However, RNA polymerase, it reads the DNA strand

in the three to the five prime direction.

Now, there's two strands that you need to be familiar with.

The first one, the one that's used to synthesize mRNA, is called the template strand

or the antisense strand.

So that's the one in which RNA polymerase is active upon.

The other one, which is not used, this is called the non-template strand,

also known as the sense strand.

Some textbooks will call it the coding strand

because its sequence matches up with RNA.

Except the fact that uracil is found in RNA, but thymine is found in DNA.

Now, during the last step of transcription, which is the termination step,

the RNA polymerase molecule, the mRNA strand, they all separate from the DNA template strand.

Now, in this step, the poly-A polymerase enzyme, it caps the three end of the mRNA strand.

And this is known as the poly-A tail.

Now, also, during the beginning of transcription,

the five end is also capped.

And the reason for this is to protect the mRNA strand from being degraded by certain enzymes.

At the end of the termination step during transcription, DNA has been used to create a pre-messenger RNA strand.

Now, this particular strand has something known as introns and exons.

Introns are basically longer sequences of nucleotides that do not code for anything.

So these, they must be removed in a process known as RNA splicing.

Now, the exons, those are shorter sequences of nucleotides,

and they're going to be used to synthesize proteins.

And so the exons, they remain, but the introns, they must be removed.

And so now we have a completed messenger RNA strand.

Now, let's work on a practice problem.

Let's say if you're given a sequence of nucleotides on a DNA strand.

And you're asked to write the corresponding sequence on an mRNA strand.

What would it be?

Feel free to pause the video and try it.

So if we're reading the DNA strand from the three to five direction,

we're going to have to write the corresponding mRNA sequence in the five to three direction.

So what letter corresponds to G?

It's important to know that G always corresponds to C.

And vice versa, C corresponds to G.

Now what letter corresponds to A?

A usually corresponds to T, but there's no T in RNA.

Instead, A is going to correspond to U for uracil.

But T in DNA corresponds to A in RNA.

And so if we continue, everything else is going to be U, C, A, U, A, U, G, C.

And so this is the mRNA strand that corresponds to the nucleotide sequence in DNA listed above.

Now, let's talk about translation,

which is the process of taking the information stored on an mRNA strand and using it to construct a protein.

So once the mRNA strand is synthesized in the nucleus, it leaves the nucleus and enters the cytosol,

where it interacts with a free ribosome or one that is attached to the rough ER.

Now, within the ribosome, it's going to interact with a tRNA molecule or a transfer RNA molecule.

Now, let's focus on the sequence of nucleotides on the mRNA strand.

Notice that they're separated in sets of three.

Each set of three nucleotides represents a codon,

which matches up with another three nucleotides on a tRNA molecule, known as an anticodon.

And each codon matches up with a specific amino acid.

And so that's how the information stored in the mRNA strand can be used to construct a specific protein.

We're going to talk more about this later in this video.

Translation, like transcription, occurs in three steps: initiation, elongation, and termination.

So here in this picture, we have a ribosome, which is composed of two subunits,

the small subunit and the large subunit.

And the ribosome has three active sites: the E site, the P site, and the A site.

Now, during initiation, we have the start codon AUG,

which corresponds to the anticodon UAC.

Now, that particular tRNA molecule has the methionine amino acid attached to it.

And this tRNA molecule, it enters the ribosome at the P site, also known as the peptidyl site,

where the peptide bonds are formed.

Now, during the second step of translation, that is during elongation,

another tRNA molecule enters the A site.

So I'm just going to draw it here.

And during that process, a covalent bond will form between the two amino acids that we see here,

highlighted by the red circles.

Now, as the process continues, the tRNA molecule in the P site

will move to the E site.

And as it does so, it's going to lose an amino acid.

And so what's going to happen is we're going to have a growing chain of amino acids

that will leave or extend out of the ribosome.

So make sure you understand that the tRNA molecules, they enter the A site,

and then they exit from the E site.

And in the process, the amino acids are being joined together.

So over time, this polypeptide chain

termination step begins when a stop codon is read.

Now, there are three stop codons that you need to be familiar with.

The first one is UAA,

and then the second one is UAG,

and the third one is UGA.

Now, these stop codons, also known as nonsense codons,

they don't code for any specific tRNA molecule.

Instead, when these are read at the A site, they cause a release factor to enter the A site,

which causes the small and the large ribosomal subunits to basically disassemble.

So this is where translation ends,

and the protein, it leaves the ribosome, where it eventually goes to the Golgi body

for further processing and modification.

At the Golgi body, the proteins undergo folding where they form a specific shape to perform a specific function.

And that's basically it for this video. Hopefully, it gave you a good overview of transcription and translation.

So that's all I got. Thanks for watching.